Optical communication or transmission systems basically comprise an optical transmitter module coupled to an optical transmission medium, such as an optical fiber, which is coupled to an optical receiver. The optical transmitter module contains a laser diode source and its circuit driver for intensity modulation of the source according to the data to be transmitted over the link. Typically, the optical signal is received at the optical receiver module where the signal is converted by a photodetector into an electrical current signal which is thereafter preamplified and, then, post-amplified. The receiver module is an important component in the data communication link since this is where noise has the greatest detrimental effect. The general aim in the design of these optical transmission systems is to optimize the signal bandwidth and minimize noise relative to a given data bit rate, but these factors are limited by the performance of the optical receiver module. Thus, in attempting to improve the signal-to-noise (SNR) performance, there are conflicting requirements for wide bandwidth and low noise generation. Therefore, it is of general desirability in system design to achieve extended receiver bandwidth without increasing the total noise.
Typical laser diodes employed in an optical transmitter have high junction temperatures, such as above 80.degree. C., so that it is necessary to provide for their effective cooling; otherwise, they will self-destruct in a short period of time. The cooling is generally accomplished by the use of thermoelectric coolers. However, these coolers are a major cause of failures in packaged laser transmitters. Such failures cannot be tolerated in many applications, such as in the case of satellite communications. Typically these coolers require 4 W of power for each watt of thermal power removed which is a major power consumption. They can typically lower the temperature of the laser diode by as much as 45.degree. C. However, this clearly is not sufficient for optical transmitter modules required to operate at temperatures in excess of 100.degree. C. Furthermore, at these high temperatures, these coolers tend to delaminate due to the low melting temperature solder employed in their manufacture. Therefore, for high temperature operational environments, the use of thermoelectric coolers is not practical. The use of uncooled laser diodes as well as compatible receiver modules at high environmental operating temperatures, such as in excess of 80.degree. C., is desired because it eliminates the requirement for thermoelectric coolers which add additional complexity and cost to the optical transmission link package due to the necessity of additional power supplies, temperature sensors and controllers.
There has been recent efforts to provide for laser diode sources that do not require cooling during operation, i.e., sources that can operate at high temperatures without with any cooling assistance, such as disclosed in the articles C. E. Zah et al., "High Performance Uncooled 1.3 .mu.m Al.sub.x Ga.sub.y In.sub.1-x-y As/InP Strained Quantum Well Lasers for Subscriber Loop Applications", IEEE Journal of Quantum Electronics, Vol. 30(2), pp. 511-523 (1994); B. Stegmuller et al., "High Temperature (130.degree.) CW Operation of 1.53 .mu.m InGaAsP Ridge Waveguide Laser Using Quaternary Quantum Wells", Electronic Letters, Vol. 29(19), pp. 1691-1693 (1993), R. Nering "Uncooled Laser Transmitter Maintains High Performance", Laser Focus World, Vol. 28(10), p. 85 et seq. (October, 1992) and P. L. Derry et al., "Low Threshold Current High Temperature Operation of InGaAs/AlGaAs Strained Quantum Well Lasers", IEEE Photonics Technology Letters, Vol. 4(11), pp. 1189-1191(1992). However, no known consideration has been given for providing a complete fiber optic data link or other optical transmission system capable of either operating uncooled at temperatures well in excess of room temperature, i.e., operating under high temperature conditions without the requirement of cooling while improving performance and operative over a bandwidth which is fairly independent of temperature over a wide range. This is particularly important where operation of the optical receiver is in an environment where low noise operation over a wide operating temperature range is a critical factor as well as insensitivity to very high levels of radiation, such as in the case of satellite communications.
Therefore, it is an object of this invention to provide a laser diode source that can operate uncooled under environmental temperatures in excess of 80.degree. C. eliminating the necessity, cost and unreliability of thermoelectric coolers.